It would be nice if play wrote the track-gain value also whenever I choose to "scan whole album" - I think this would be easy to do because the values are scanned anyway. this is because I like to choose whether I want to hear shuffeled songs or whole albums from time to time.

Play should always scan track gain when calculating replay gain (album or track). Are you observing different behavior? To see what Play thinks the RG values for a track are, check in the Track Info window under the Replay Gain tab.